Lysosomal storage diseases (LSDs) are a group of over 60 different rare, inherited metabolic diseases caused by gene mutations that impair lysosomal function and homeostasis (Schultz et al. 2011; Lieberman et al. 2012). They typically present in childhood, collectively affecting 1 in 7,000 live births, of which most are fatal at young age. Lysosomes are membrane-enclosed acidic cytoplasmic organelles that contain about 60 specialized acid hydrolases to break down all types of biological macromolecules (aka cellular waste) including proteins, nucleic acids, lipids and carbohydrates (Saftig 2006). Genetic mutations of genes encoding lysosomal enzymes such as hydrolases or associated proteins cause blockages in stepwise degradations of those cellular macromolecules (substrates), leading to their accumulation or partial accumulation in the lysosomes. The intra-lysosomal accumulation of non-degraded substrates will affect multiple tissue and organ systems including brain, bone, heart and viscera. Early onset central nervous system dysfunction predominates most of LSDs, with various progressive clinical phenotypes. All LSDs are recessively inherited monogenic disorders, and a few are X-linked inheritance such as Fabry and Hunter diseases (Desnick et al. 2003; de Camargo Pinto et al. 2011).
LSDs can be categorized based on the type of macromolecules (lipid, mucopolysaccharidoses, glycoprotein, etc.) that fail to be degraded and are consequently stored (Schultz et al. 2011). Sphingolipids are one type of lipid macromolecules that play significant roles in cell signaling, and regulate cell function through their bioactive metabolites. The synthesis and degradation of sphingolipids are governed by a cascade of enzymatic reactions in common synthetic (in ER) and catabolic (in lysosomes) pathways. Three major rate limiting enzymes in sphinogolipid biosynthesis are ceramide galactosyltransferase (CGT), glucosylceramide synthase (GCS) and sphingomyelin synthase (Gault, Obeid, and Hannun 2010). They generate precursors from ceramide for galactosphingolipids, glucosphigolipids and sphingomyelin, respectively, in ER and Golgi compartments. Glucosylceramide is the precursor for majority of all glycosphingolipids that are essential for cell-cell recognition during development. On the other hand, those complex sphingolipids are constantly catabolized into ceramide for recycling and preventing lipid accumulation in the cells that are toxic to proper cell functions. Glycosphinolipid hydrolases are needed for glycosphingolipid catabolism. Mutations of genes encoding those glycohydrolases including glucocerebrosidase (GBA1) and alpha-galactosidase A (GLA) are the mechanisms underlying most common lipid storage diseases.
Gaucher and Fabry diseases carrying mutations on GBA1 and GLA, respectively, represent the prototypic sphinogolipid storage disorders. Gaucher disease is the most common lysosomal storage disease with incidence of 1 in 40,000 for non-Jewish population. It is an autosomal recessive disorder caused by mutations in the GBA1 gene. GBA1 encodes for beta-glucocerebrosidase that degrades glucosylceramide to ceramide and glucose (Mistry et al. 2017). Its deficiency causes accumulation of glucoceramide in macrophages of the spleen, liver, bone marrow and lung. Patients suffer from significant anemia, bone lesion, hepatosplenomegaly, thrombocytopenia and growth retardation. Most of the Gaucher disease alleles are GBA missense mutations that lead to the synthesis of acid beta-glucosidases with decreased catalytic function and/or stability; whereas nonsense/frameshift mutations in GBA genes have also been reported. GBA mutations are also strong risk factors for neurologic disorders like Parkinson disease (PD) and Dementia with Lewy bodies (DLB), with 2.3-9.4% of PD patients carrying a GBA mutation (Balestrino and Schapira 2018). Fabry disease, on the other hand, is an X-linked sphinogolipid storage disease due to the loss (majority cases) of alpha-galactosidase activity (Chan and Adam 2018). Its prevalence ranges from 1 in 50,000 to 1 in 117,000. The causative gene for Fabry disease is GLA located to chromosome Xq22. The loss of enzymatic activity leads to the systemic deposition of glycosphingolipid substrates in a number of cell types in the heart, kidney, eyes and other tissues. Death usually occurs from renal failure or cardiac/cerebrovascular disease.
Because of the monogenic nature of LSDs, enzyme replacement therapy (ERT) has become the first line of treatment for a number of LSDs, using purified recombinant enzymes to replace the defective enzymes in the lysosome and therefore reducing pathological substrate accumulation in the lysosome. Imiglucerase, taliglucerase and velaglucerase have been used for treating Gaucher disease; alglucosidase alfa for Pompe disease and agalsidase alfa/beta for Fabry disease (Desnick and Schuchman 2012; Parenti, Andria, and Valenzano 2015). ERT is administrated through direct intravenous infusion of recombinant enzymes to patients with inherited deficiencies. Even though it has been shown to successfully treat certain LSDs, it has limitations. First, it requires life-time intravenous infusion every 2 weeks, posing a high cost for treatment. Second, the tissue penetration of the enzymes varies among organs, with low penetration to the bones, lung and brain. Third, the efficacy of ERT is often reduced due to the unwanted immune response against the enzyme. Due to these drawbacks of ERT, substrate reduction therapy (SRT) using small molecules (Platt et al. 1994; Patterson et al. 2007; Lukina et al. 2010) or small interfering RNAs (Canals et al. 2015) has emerged as a second line of therapy for LSDs. SRT does not target the mutant enzyme; instead, it prevents/reduces the synthesis of substrates using small molecules to inhibit key enzymes in substrate biosynthesis pathways. It is a non-disease-specific LSD therapy, as all GSLs except galactosylceramide are synthesized through a common biosynthetic pathway with GCS as a rate limiting enzyme. This GCS synthase is the target for two currently approved drugs for treating a number of LSDs: miglustat (Machaczka et al. 2012) (Zavesca; Actelion Pharmaceuticals) and eliglustat (Lukina et al. 2010) (Cerdelga; Genzyme).
Miglustat was the first oral small molecule therapeutic drug approved by FDA in 2003 for treating moderate type 1 Gaucher disease that is not suitable for ERT. It is an imino sugar drug with glucose stereochemistry that acts as a GCS inhibitor. However, its serious side effects such as pain, burning, neurological, vision and gastrointestinal discomfort limits its clinical use (Remenova et al. 2015). Eliglustat, a recently approved small molecule by FDA in 2014 has become a first-line oral therapy for type 1 Gaucher disease. It is a stronger GCS inhibitor with equivalent efficacy to ERT and less severe side effects. However, it is also a substrate for CYP2D6, therefore requiring individual adaptation of the dose to be effective. In addition, Eliglustat inhibition carries impact on hundreds of different glycoproteins, leading to high cardiac adverse effect incident and increased risk of arrhythmia. Eliglustat has poor penetration across the blood-brain barrier, making it unsuitable for treating CNS related neurological disorders in GSL storage diseases (Mistry et al. 2015). Therefore, there is an urgent and growing need to develop better inhibitors for SRT. ibiglustat (Venglustat; Genz-682452; GZ/SAR402671), a potent and selective GCS inhibitor, showes great potential in complement and augment ERT therapies (Ashe et al. 2015; Marshall et al. 2016). It has low toxicity and is capable of traversing the blood-brain barrier, leading to improved clinical effectiveness with reduced side effects. Currently, ibiglustat is in phase II trials for Fabry, Gaucher and Parkinson diseases.
To this end, structural modification of existing drugs or potential drug-able small molecules has been playing a significant role in generating new chemical entities that are biologically potent and physiologically active with improved pharmacokinetic, therapeutic, and toxicological profiles. One such methodology that has been attempted is to use deuteration substitution as a tool for optimization of drug metabolic profiles and reduce toxicity (Harbeson et al. 2017; Timmins 2014; Russak and Bednarczyk 2018).